1. Field of the Invention
The present invention relates to fatigue failure diagnostic method and apparatus for a turbocharger mounted on an engine.
2. Description of the Related Art
A turbocharger (supercharger) comprises a turbine connected to the exhaust gas channel of an engine and driven by the exhaust gas of the engine and a compressor connected to the intake channel of the engine and driven by the turbine. The turbine comprises a turbine wheel fixedly mounted on a rotary shaft. The compressor comprises a compressor impeller fixedly mounted on the same rotary shaft as the turbine wheel. The compressor impeller located on the same rotary shaft is rotated by rotating the turbine wheel with the exhaust gas of the engine. As a result, the compressor intakes the air and the pressure of the intake air is increased. Further, the intake air under increased pressure is supplied to the engine.
Because the compressor impeller of the turbocharger rotates at a very high speed, a comparatively large load is applied to the compressor impeller. If the compressor impeller is fractured, the fractured pieces thereof can be sucked into the engine. For this reason the replacement period of the compressor impeller is determined in advance and the compressor is replaced after each such replacement period.
With the conventional method for diagnosing the turbocharger fatigue, the degree of fatigue (in particular, LCF (Low Cycle Fatigue)) accumulated in the compressor impeller was evaluated based on the empiric rule, experiment, or analysis and the replacement period was determined based on the estimation results. For example, the estimation of fatigue was conducted based on the test data on the revolution speed of the compressor impeller that assumed the operation state of the engine.
Japanese Patent Application Laid-open No. 2001-329856 described a method for diagnosing the fatigue of a gas turbine. This method comprises the steps of measuring pressure fluctuations at the blade stage of a gas turbine compressor, conducting stress analysis by using the measured pressure fluctuation data and structure analysis model of the compressor blades and estimating the stress fluctuations in the actual operation environment of the compressor blades, comparing the stress fluctuations of the compressor blades that were thus estimated with the strength master curve under corrosive environment of the compressor blade material, evaluating the fatigue damage of the compressor blades, and determining the replacement period of the compressor blades based on the evaluated fatigue damage.
However, vehicles carrying the engines are used in a variety of different ways and the degree of fatigue accumulated in each compressor impeller can vary significantly. Therefore, the replacement period relating to all the actual operation states of the engine is difficult to determine. For example, when an engine is operated at a comparatively high altitude or with a comparatively high acceleration and deceleration frequency, the fatigue is comparatively rapidly and easily accumulated in the compressor impeller and the compressor impeller has to be replaced before the replacement period elapses. Furthermore, if the replacement period is determined to match an unnecessarily severe operation mode, the replacement is conducted before it is actually necessary, thereby increasing the cost.